The present invention relates generally to the abatement of contaminant laden industrial process gases and more particularly, to an abatement system which utilizes a rotary regenerative oxidizer.
Industrial process gases typically contain particulates and major gaseous air pollutants such as volatile organic compounds (VOCs) and carbon monoxide (CO), which contaminate the environment when vented to the atmosphere. Thermal and catalytic oxidizers are generally used to eliminate these contaminants. Thermal oxidizers utilize a supplementary heat source to increase the temperature of the inlet process gas to a level above the ignition temperature of the combustible contaminants, generally ranging from 1400.degree. F. (.apprxeq.760.degree. C.) to 1800.degree. F. (.apprxeq.980.degree. C.), so as to oxidize combustible contaminants such as VOCs and CO. Catalytic oxidizers further utilize a catalytic material to effect oxidation at lower peak temperatures.
Regenerative thermal and catalytic oxidizers (RTOs and RCOs) recover heat remaining in the cleansed exhaust gas, and through heat exchange, increase the temperature of gases entering the oxidizer thereby minimizing the amount of supplemental energy required to bring the gas to its ignition temperature. RTOs and RCOs generally operate in cycles and comprise a plurality of regenerative beds and a corresponding number of catalytic beds if an RCO is utilized. Characteristically, flow control valves are used to direct the inlet process gases to one or more regenerators for preheating prior to thermal or catalytic oxidation. RCOs and RTOs are generally of a fixed-bed design wherein the unit remains stationary as the process gases pass through for purification. One related concern is the relatively large floor space occupied by such a fixed-bed arrangement.
Catalytic oxidation, in contrast to thermal oxidation, reduces energy costs by lowering the reaction temperatures. Catalytic reaction temperatures typically range from 200 to 400.degree. C. as compared to 700 to 980.degree. C. required in thermal incinerators. However, catalytic reaction temperatures are still substantially greater than most flue gas temperatures and therefore, heat exchangers are frequently used as part of a control system to further lower the energy costs.
Regenerative or recuperative heat exchangers are commonly used in both RTOs and RCOs. Regenerative heat exchanger systems have a relatively higher thermal efficiency, typically 70 to 95%, while recuperative systems achieve at best 70% thermal efficiency. Most regenerative systems incorporate flow reversal design combined with heat sink packing material to achieve an optimum thermal efficiency. However, regenerative heat exchangers are expensive to implement and therefore, are generally only used for industrial process flow rates of approximately 850 Nm.sup.3 /minute or 30,000 SCFM (standard cubic feet per minute), or more. Recuperative heat exchangers are less expensive to implement, and thus are incorporated with industrial process flow rates of approximately 3 Nm.sup.3 /minute--850 Nm.sup.3 /minute or about 100--30,000 SCFM.
In a fixed-bed regenerative unit, the polluted gases first flow through an input heat regenerative bed, and then are oxidized before exiting the unit through an output heat regenerative bed. Heat absorbed by the process gases is transferred to the output heat regenerative bed for preheating of process gases subsequently entering the bed. Valve design facilitates process gas flow reversal and preheating thereof by directing the polluted influx into regenerative heat beds that functioned as output regenerative beds in the prior treatment cycle. A related concern, however, is that the characteristic flow reversal causes pressure fluctuations that may adversely affect the upstream process conditions. Furthermore, fixed-bed systems experience pressure drops, and as such, increased energy costs in order to maintain a consistent linear gas velocity.
Recuperative systems generally use a heat transfer surface as a medium to transfer heat, for example, in plate or shell designs. In addition to limited thermal efficiency, recuperative systems have associated mechanical concerns including condensation and corrosion. Condensation and corrosion are caused by uneven heating which forms "cold spots" on the heat exchange surface. Furthermore, use of a recuperative heat exchanger results in excess energy consumption due to high pressure drops across the heat exchange surface.